Chlorine dioxide (ClO.sub.2) is a highly reactive gas that is used as a substitute for chlorine, which is rapidly being phased out of many industries, such as pulp and paper, flour and textiles, due to environmental concerns regarding the formation of dioxins. In addition to being used as a bleaching agent, chlorine dioxide is also used for other purposes such as the disinfection and sterilization of foods and drinking water, and for the treatment of leather. Chlorine dioxide is so highly reactive that it cannot easily be stored in compressed gas cylinders, and must be generated at the point of use. Chlorine dioxide is commonly generated by the electrochemical oxidation of chlorite salts. Further details of the chemistry of chlorine dioxide may be found in standard texts, such as "Chlorine Dioxide--Chemistry and Environmental Impact of Oxychlorine Compounds," W. J. Masschelein, Ann Arbor Science Publishers Inc, Ann Arbor, Mich.
This high level of reactivity is also reflected in a high toxicity, and the OSHA work place permissible exposure to chlorine dioxide is only 0.1 ppm averaged over an eight hour shift, with a level of 5 ppm considered immediately dangerous to life and health (NIOSH Pocket Guide to Chemical Hazards, U.S. Department of Health and Human Services, June 1997). Thus, it is important to have adequate protection of personnel who are using chlorine dioxide. While most facilities using chlorine dioxide gas use engineering controls to ensure that the ambient concentration of chlorine dioxide is maintained at safe levels, leaks and other emissions unfortunately do occur, posing a risk to personnel in the vicinity. Therefore, it is common practice to use gas detection instruments to monitor for chlorine dioxide and other potentially hazardous gases.
These instruments may be portable instruments intended to provide personal protection, and are typically worn by the personnel to be protected. Alternatively, fixed (e.g. wall mounted) gas detection devices may be employed which monitor the area for the presence of potentially hazardous atmospheres. In a typical application such as a paper mill, instruments may be used to detect chlorine dioxide, hydrogen sulfide, sulfur dioxide and oxygen deficiency. In many cases, multi-gas instruments are available which incorporate sensors for several different types of gases.
Hydrogen sulfide is typically found in many industrial applications, including petroleum-refining operations, coking of coal, purification of natural gas and the evaporation of black liquor in Kraft pulping. For large-scale operations, the hydrogen sulfide is recovered and converted to sulfur dioxide for subsequent conversion to sulfuric acid or elemental sulfur. For smaller scale operations, other pollution control processes are used, such as iron-oxide fire boxes, wet scrubbers containing solutions of oxidants such as chlorine, alkaline potassium permanganate or atmospheric oxygen, or bases such as organic amines, (e.g. ethanolamine) and tripotassium phosphate and sodium carbonate ("Industrial Pollution control Handbook", H. F. Lund, Ed., McGraw-Hill book Company, New York, 1971; "Pollutant Removal Handbook", M. Sittig, Noyes Data Corporation, Park Ridge, N.Y., 1973; Kirk-Othmer Encyclopedia of Chemical Technology, 2nd Ed., Vol. 19, Interscience Publishers, New York, 1969). More recently, a number of cobalt, iron and manganese chelate and macrocycle compounds have also been used to catalyze the air oxidation of hydrogen sulfide. Mimoun et al in U.S. Pat. No. 3,956,473 and Deberry et al in U.S. Pat. No. 5,705,135, describe the use of iron chelates in non-aqueous solvents. Anderson et al in U.S. Pat. No. 3,923,645 describe the use of substituted cobalt porphyrins as catalysts for the air oxidation of hydrogen sulfide. Analogously, Verachtert in U.S. Pat. No. 5,244,643 describes the use of transition metal phthalocyanine complexes as catalysts for the air oxidation of hydrogen sulfide and mercaptans in aqueous alkali. Bridges et al in U.S. Pat. No. 5,527,517 have also described the removal of hydrogen sulfide from a gas stream by oxidation of the hydrogen sulfide by aqueous hydrogen peroxide, catalyzed by a silver nitrate or other silver salts.
Hydrogen sulfide gas is of particular concern in those locations where both hydrogen sulfide and chlorine dioxide may be found. Typical locations include pulp and paper mills, water treatment plants, etc. Hydrogen sulfide is a toxic gas, the OSHA permissible exposure limit having a ceiling value of 10 ppm and a level considered immediately dangerous to life and health (IDLH) of 100 ppm (NIOSH Pocket Guide to Chemical Hazards, U.S. Department of Health and Human Services, June 1997). Thus, although hydrogen sulfide is a highly toxic gas, it is less toxic than chlorine dioxide.
In electrochemical gas sensors, the response to hydrogen sulfide is typically the eight-electron oxidation hydrogen sulfide to sulfuric acid. EQU H.sub.2 S+4H.sub.2 O.fwdarw.H.sub.2 SO.sub.4 +8H.sup.+ +8e.sup.-
The response of an electrochemical sensor to chlorine dioxide is typically either the one electron reduction to hydrogen chlorite: EQU ClO.sub.2 +H.sup.+ +e.sup.-.fwdarw.HClO.sub.2
or the five electron reduction to chloride ion: EQU ClO.sub.2 +4H.sup.+ +5e.sup.-.fwdarw.2H.sub.2 O+Cl.sup.-
depending on the electrodes and electrolytes used in the sensor. Since hydrogen sulfide is less toxic than chlorine dioxide, a concentration of hydrogen sulfide, within the permissible exposure levels, maybe larger than the permissible exposure levels for chlorine dioxide. Furthermore, the hydrogen sulfide will give a much larger response from the electrochemical gas sensor per unit concentration (eight electrons for hydrogen sulfide versus one electron for chlorine dioxide).
In view of the ease with which hydrogen sulfide is oxidized, electrochemical sensors are often designed to have a reduced sensitivity to hydrogen sulfide compared to the response expected based on gas diffusion of hydrogen sulfide. Despite the efforts of sensor designers, there is still usually a significant response from the sensors to hydrogen sulfide.
However, the most serious problem from a safety perspective is that the response to hydrogen sulfide in an electrochemical sensor is an oxidation reaction, whereas the response to chlorine dioxide in an electrochemical sensor is a reduction reaction. Exposure of a chlorine dioxide detection instrument to hydrogen sulfide alone will usually give a negative response. The response to hydrogen sulfide is in the opposite polarity to the response to chlorine dioxide, and thus the sum of the responses of chlorine dioxide and hydrogen sulfide will be less than that of the same concentration of chlorine dioxide on its own.
Hydrogen sulfide also responds on other electrochemical gas sensors; unfiltered sensors for carbon monoxide, sulfur dioxide, nitrogen dioxide, chlorine, hydrogen, hydrogen chloride and ammonia from, for example, City Technology Ltd., one of the largest gas sensor manufacturers, all give a responses to hydrogen sulfide (Product Data Handbook, Vol. 1. Issue 4, Safety, City Technology Ltd., Portsmouth, United Kingdom, June 1997).
Many sensors incorporate chemical filters in an attempt to reduce the cross sensitivity to hydrogen sulfide; however chemical filters cannot be used for all types of gas sensors, since the filter must scrub out the unwanted gas, but still let the analyte gas pass through to the sensor electrodes. The use of chemical filters within sensors is well known in the prior art; for example, Tantam and Chan in U.S. Pat. No. 4,633,704 describe the use of a soda lime filter to prevent hydrogen sulfide from giving a response on a carbon monoxide sensor.
Carbon filters are also commonly used to protect gas sensors from hydrogen sulfide, as is illustrated by Kiesele et al in U.S. Pat. No. 5,865,973, Xu in U.S. Pat. No. 5,803,337 and Martell et al in U.S. Pat. No. 5,744,697. The use of carbon filters is restricted to only a few types of gas sensors, since activated carbon absorbs a wide range of gases.
One common type of filter employs an oxidizing agent, such as potassium permanganate. Commercially available potassium permanganate impregnated filter media for air filtration are available from companies such a Purafil Inc., Doraville, Ga. Potassium permanganate is a strong oxidizing agent, and oxidizes any hydrogen sulfide which comes into contact with the filter medium, thus preventing it from entering the sensor. Since potassium permanganate is such a strong oxidizing agent, it will remove most easily oxidizable gases, including chlorine dioxide. Thus it is only suitable for relatively few types of gas sensors, such as those for carbon monoxide.
Chlorine commonly exists in oxidation states from -1 to +7, so chlorine dioxide, with chlorine in oxidation state +4 can be both oxidized and reduced, as is illustrated by the following redox reactions (CRC Handbook of Chemistry and Physics, 68.sup.th Edition, 1987-1988, R. C. Weast, M. J. Astle, W. H. Beyer, Eds.):
Reduction: ClO.sub.2 +H.sup.+ +e-.revreaction.HClO.sub.2 E=1.277V PA1 Oxidation: ClO.sub.2 +H.sub.2.revreaction.ClO.sub.3.sup.- +2H.sup.+ +e-E=1.152V [Note that the redox potentials are written as reduction potentials, even for the oxidation reactions.]
Chlorine dioxide can be oxidized and reduced to many other species in addition to the two illustrative reactions depicted above. Thus, exposure of chlorine dioxide to strong oxidizing agents will result in the oxidation of the chlorine dioxide, and prevent it from reaching the sensor.
Another type of filter commonly used to protect gas sensors from exposure to hydrogen sulfide uses a metal salt which forms an insoluble sulfide. One of the most commonly used metal salts is lead acetate, which forms the black product, lead sulfide, upon exposure to hydrogen sulfide. EQU Pb(O.sub.2 CCH.sub.3).sub.2 +H.sub.2 S.fwdarw.PbS+2HO.sub.2 CCH.sub.3
Blurton et al in U.S. Pat. Nos. 4,001,103, 4,052,268 and 4,127,462 describe the use of lead acetate and mercuric chloride filters to protect nitrogen dioxide and nitric oxide electrochemical sensors from hydrogen sulfide. Similarly, Stahl et al have described the use of some silver salts for preventing the cross sensitivity of hydrogen sulfide on sulfur dioxide sensors in U.S. Pat. No. 4,127,386.
Exposure of aqueous solutions of many transition metal ions to hydrogen sulfide results in the precipitation of the darkly colored metal sulfides. These reactions are well known in analytical chemistry and are used as a means of identifying the metal ions, as well as a test for hydrogen sulfide in spot tests and gas detection tubes ("Inorganic Reactions at Advanced Level", D. G. Davies, T. V. G. Kelly, Mills & Boon Ltd., London, 1977; "Spot Tests", F. Feigl, Vol. 1, Elsevier Publishing Company, Amsterdam, 1954; "Drager-Tube Handbook", 8.sup.th Edition, National Draeger Inc., Pittsburgh, Pa.). Silver nitrate, for example, has been used to measure hydrogen sulfide concentration in atmospheric studies using impregnated paper tape ("Hydrogen sulfide in the atmosphere of the northern equatorial Atlantic ocean and its relation to the global sulfur cycle," B. J. Slatt, D. F. S. Natusch, J. M. Prospero, D. L. Savoie, Atmos. Environ. (1978), 12 (5), 981-991; "Determination of hydrogen sulfide in air: an assessment of impregnated paper tape methods", D. F. S. Natusch, J. R. Sewell, R. L. Tanner, Analytical Chemistry, (1974), 46 (3), 410-415). A more complete discussion of the reactions of hydrogen sulfide with transition metals can be found in standard texts ("Inorganic Chemistry," P. C. L. Thorne, E. P. Roberts, Interscience Publishers Inc., New York, 1948).
The use of filters containing lead, mercury and other transition metals to prevent the cross sensitivity of hydrogen sulfide on gas sensors is well known in the prior art, as is illustrated by the patents of Blurton et al described above. This type of filter usually works very well for many types of sensors and other applications where it is necessary to remove hydrogen sulfide gas. Lead sulfide filters have even been used in prior art sensors for chlorine dioxide, though their use in this application has severe drawbacks.
Chlorine dioxide does not react with lead acetate to a significant extent, and thus passes through a lead acetate impregnated filter placed in the gas diffusion path of the sensor. Hydrogen sulfide reacts with lead acetate to form lead sulfide and thus the hydrogen sulfide is unable to reach a sensor protected by a lead acetate impregnated filter. However, it has been found that this lead sulfide formed by the reaction of the hydrogen sulfide with the lead acetate reacts readily with chlorine dioxide, so if a sensor protected by a lead acetate impregnated filter is sequentially exposed to test gases containing chlorine dioxide, then hydrogen sulfide and then chlorine dioxide again, the response of the sensor to the second exposure to chlorine dioxide is greatly reduced compared to the response to the first exposure to chlorine dioxide.
This particular problem is especially insidious since a sensor protected with a metal acetate filter may respond well to chlorine dioxide prior to exposure to hydrogen sulfide. There may be no indication of exposure to hydrogen sulfide, since the filter is effective at removing that gas, but subsequent to the exposure of hydrogen sulfide, the filter will greatly reduce the amount of chlorine dioxide reaching the sensor. Alternatively, in an environment such as a paper mill where exposure to low levels of hydrogen sulfide (below permissible exposure limits) is common, a gradual loss in response to chlorine dioxide may occur, and the instrument may fail to provide a warning to personnel in the event of a dangerous chlorine dioxide atmosphere. The hydrogen sulfide exposure need not be at the same time as a the chlorine dioxide exposure; instead it may have occurred at an earlier time, or even another day.
Clearly, there is a need for a filter that is suitable for use with an electrochemical gas sensor which will prevent hydrogen sulfide from reaching the sensor, and which will allow chlorine dioxide to reach the gas sensor both prior to and subsequent to exposure to hydrogen sulfide.